Ethanol for industrial use is conventionally produced from organic feed stocks, such as petroleum oil, natural gas, or coal, from feed stock intermediates, such as syngas, or from starchy materials or cellulose materials, such as corn or sugar cane. Conventional methods for producing ethanol from organic feed stocks, as well as from cellulose materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in organic feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulose materials, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol, which is suitable for fuels or human consumption. In addition, fermentation of starchy or cellulose materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.
Ethanol production via the reduction of alkanoic acids and/or other carbonyl group-containing compounds, including esters, has been widely studied, and a variety of combinations of catalysts, supports, and operating conditions have been mentioned in the literature.
More recently, even though it may not still be commercially viable it has been reported that ethanol can be produced from hydrogenating acetic acid using a cobalt catalyst at superatmospheric pressures such as about 40 to 120 bar, as described in U.S. Pat. No. 4,517,391.
On the other hand, U.S. Pat. No. 5,149,680 describes a process for the catalytic hydrogenation of carboxylic acids and their anhydrides to alcohols and/or esters utilizing a platinum group metal alloy catalyst. The catalyst is comprised of an alloy of at least one noble metal of Group VIII of the Periodic Table and at least one metal capable of alloying with the Group VIII noble metal, admixed with a component comprising at least one of the metals rhenium, tungsten or molybdenum. Although it has been claimed therein that improved selectivity to a mixture of alcohol and its ester with the unreacted carboxylic acid is achieved over the prior art references it was still reported that 3 to 9 percent of alkanes, such as methane and ethane are formed as by-products during the hydrogenation of acetic acid to ethanol under their optimal catalyst conditions.
A slightly modified process for the preparation of ethyl acetate by hydrogenating acetic acid has been reported in EP0372847. In this process, a carboxylic acid ester, such as for example, ethyl acetate is produced at a selectivity of greater than 50% while producing the corresponding alcohol at a selectivity less than 10% from a carboxylic acid or anhydride thereof by reacting the acid or anhydride with hydrogen at elevated temperature in the presence of a catalyst composition comprising as a first component at least one of Group VIII noble metal and a second component comprising at least one of molybdenum, tungsten and rhenium and a third component comprising an oxide of a Group IVB element. However, even the optimal conditions reported therein result in significant amounts of by-products including methane, ethane, acetaldehyde and acetone in addition to ethanol. In addition, the conversion of acetic acid is generally low and is in the range of about 5 to 40% except for a few cases in which the conversion reached as high as 80%.
Copper-iron catalysts for hydrogenolyzing esters to alcohols are described in U.S. Pat. No. 5,198,592. A hydrogenolysis catalyst comprising nickel, tin, germanium and/or lead is described in U.S. Pat. No. 4,628,130. A rhodium hydrogenolysis catalyst that also contains tin, germanium and/or lead is described in U.S. Pat. No. 4,456,775.
Several processes that produce ethanol from acetates, including methyl acetate and ethyl acetate, are known in the literature.
WO8303409 describes producing ethanol by carbonylation of methanol by reaction with carbon monoxide in the presence of a carbonylation catalyst to form acetic acid which is then converted to an acetate ester followed by hydrogenolysis of the acetate ester formed to give ethanol or a mixture of ethanol and another alcohol which can be separated by distillation. Preferably the other alcohol or part of the ethanol recovered from the hydrogenolysis step is recycled for further esterification. Carbonylation can be effected using a CO/H2 mixture and hydrogenolysis can similarly be conducted in the presence of carbon monoxide, leading to the possibility of circulating gas between the carbonylation and hydrogenolysis zones with synthesis gas, preferably a 2:1 H2:CO molar mixture being used as makeup gas.
WO2009063174 describes a continuous process for the production of ethanol from a carbonaceous feedstock. The carbonaceous feedstock is first converted to synthesis gas which is then converted to ethanoic acid, i.e., acetic acid, which is then esterified to form ethyl acetate. The ethyl acetate stream is then purified to remove the unreacted acetic acid and the purified stream is then hydrogenated to produce ethanol. The purified stream preferably contains less than 5 wt. % acetic acid because it is know that high levels of acetic acid deactivates copper catalysts and renders them ineffective in converting ethyl acetate to ethanol. The reference further disclose the use of a copper based catalyst comprises between 20 and 40 wt. % copper on a zinc oxide support.
WO2009009320 describes an indirect route for producing ethanol. Carbohydrates are fermented under homoacidogenic conditions to form acetic acid. The acetic acid is esterified with a primary alcohol having at least 4 carbon atoms and hydrogenating the ester to form ethanol.
EP2060555 describes a process for producing ethanol where a carbonaceous feedstock is converted to synthesis gas which is converted to ethanoic acid, which is then esterified and which is then hydrogenated to produce ethanol. EP2072489 and EP2186787 also describe a similar process where the esters produced from esterification are fed to the alcohol synthesis reactor used to produce ethanol and methanol.
US Pub. No. 20110046421 describes a process for producing ethanol comprising converting carbonaceous feedstock to syngas and converting the syngas to methanol. Methanol is carbonylated to ethanoic acid, which is then subjected to a two stage hydrogenation process. First the ethanoic acid is converted to ethyl ethanoate followed by a secondary hydrogenation to ethanol.
US Pub. No. 20100273229 describes a process for producing acetic acid intermediate from carbohydrates, such as corn, using enzymatic milling and fermentation steps. The acetic acid intermediate is acidified with calcium carbonate and the acetic acid is esterified to produce esters. Ethanol is produced by a hydrogenolysis reaction of the ester.
U.S. Pat. No. 5,414,161 describes a process for producing ethanol by a gas phase carbonylation of methanol with carbon monoxide followed by a hydrogenation. The carbonylation produces acetic acid and methyl acetate, which are separated and the methyl acetate is hydrogenated to produce ethanol in the presence of a copper-containing catalyst.
U.S. Pat. No. 4,497,967 describes a process for producing ethanol from methanol by first esterifying the methanol with acetic acid. The methyl acetate is carbonylated to produce acetic anhydride which is then reacted with one or more aliphatic alcohols to produce acetates. The acetates are hydrogenated to produce ethanol. The one or more aliphatic alcohols formed during hydrogenation are returned to the acetic anhydride esterification reaction.
U.S. Pat. No. 4,454,358 describes a process for producing ethanol from methanol. Methanol is carbonylated to produce methyl acetate and acetic acid. The methyl acetate is recovered and hydrogenated to produce methanol and ethanol. Ethanol is recovered by separating the methanol/ethanol mixture. The separated methanol is returned to the carbonylation process.
The need remains for improved processes for efficient ethanol production by reducing esters on a commercially feasible scale.